The present invention relates to a combination fuse, especially for fire protection installations, comprise two metal pieces which normally are bonded together by means of a fusible metal and during normal condition prevent a trip mechanism to be operated, whereas at a temperature above normal ambient temperature the fusible metal loses its bonding effect and releases the trip mechanism, and a heat dissipating means adapted to dissipate heat so as to raise the temperature of the fusible metal for releasing the combination fuse, the heat dissipation being initiated in response to a signal generated by the sensing of local or remote ambient conditions.
In connection with fire protection installations which may include sprinkler devices and/or fire dampers and the like, there may in addition to a local detection and release of the fire protection devices be a demand for remote control of several or groups of such devices. The remote control may be manually initiated from a control station or automatically from detection means responsive to smoke, heat or other ambient conditions and signalling existing or approaching fire. By means of the remote control it is possible selectively to close off the area which is hit or threatend by fire and thus more effectively to utilize the fire protection devices included in larger installations.
The remote control of the devices included in a fire protection installation, has up till now been carried out by means of electro-magnetic components. Such components are very expensive, require a large installation space and suffer from certain drawbacks as regards reliability of operation.
From U.S. Pat. No. 2,272,857 there is known a fuse element which primarily has found application in sprinkler installations. The fuse element is connected to an electric installation by means of two conductors, one of which is connected to the sprinkler head, i.e., the technical installation. The use of the technical installation as a current path is, however, very unfavable, the electrical resistance value given by such an installation being prone to changes, inter alia due to corrosion, and hence very difficult to calculate accurately. In the fuse element according to this patent specification the fuse element proper constitutes the resistance element which, when heated to a desired temperature, will melt the metal of the element. The resistance of the fuse element will together with the resistances afforded by the technical installation and the transition resistances therebetween, give a measure for the voltage necessary to be applied in order to pass through the fuse element a current having an intensity sufficient for making the fusion metal lose its holding ability. In practice this involves such a high voltage that special precautions have to be taken for protection against touch.
In GB Pat. No. 1,406,677 there is disclosed a fuse element which like the fuse element according to the patent specification discussed above, finds application in sprinkler installations. The fuse element according to GB Pat. No. 1,406,677 comes into operation when the ambient temperature exceeds a given value, and comprises heat generating means which in response to an electric signal blows an open flame towards the fusible metal of the fuse element so as to deprive it of its holding ability. However, the fuse according to GP Pat. No. 1,406,677 does not render any possibility of checking whether or not the fuse element is intact.
From G Pat. No. 1,363,945 there is known a fuse element consisting of two rupturable bulbs which in addition to being ruptured due to an increased ambient temperature, may also be ruptured by means of a piston-operated striker which is discharged towards the bulbs by means of an explosive charge which is ignited by an electric pulse.
In U.S. Pat. No. 3,897,828 there is described a fuse element consisting of two metal pieces which are bonded together by a fusible metal, which during normal conditions prevents a trip mechanism from being actuated. The known element comprises a filament which may be a single or multi-stranded lead passing from an area to be protected by the fire extinguishing apparatus holding the fuse element. Thus, the fuse element according to U.S. Pat. No. 3,897,8282 is involved in a direct transfer of heat via a conductor from a heated area to the element, the heat being transferred via the filament.
Finally, from U.S. Pat. No. 1,029,277 there is known a fire extinguishing apparatus comprising a heat filament which is connected to a current source by two leads. The one lead is, however, connected to ground, and also in such a fire extinguishing apparatus, one will encounter the same problems with respect to incalculable resistance of the current path as discussed above in connection with U.S. Pat. No. 2,272,857.